A magnet arrangement in a magnetic resonance apparatus known in the art has a permanent magnet system for generating a homogeneous magnetic field in a direction perpendicular to a z axis in a measurement volume. This permanent magnet system includes at least 2, in particular at least 3, ring-shaped magnet elements made of magnetic material, which are arranged cylindrically symmetrically with respect to the z axis and stacked on one another in the z direction and/or concentrically. The ring-shaped magnet elements are made up of individual magnet segments and are arranged such that the magnetization direction of the individual segments in the respective rings extends essentially parallel in an x-y plane perpendicular to the z direction. The ring-shaped magnet elements respectively have a Halbach magnetization that generates a magnetic dipole field, and the ring-shaped magnet elements are alignable relative to one another in the z direction.
Such a magnet arrangement is known from EP 2 144 076 B1.
Both in the field of nuclear magnetic resonance (NMR) spectroscopy and in imaging applications (MRI), a very homogeneous magnetic field which is constant as a function of time is required in a sample volume to be defined. This field may be generated with resistive or superconducting coils or a suitable permanent magnet arrangement. The use of permanent magnets is preferred when flux densities of less than 2 T are sufficient and a relatively compact structure is desired.
In order to maximize the magnetic flux and the sample volume, and therefore at the same time to minimize the stray flux, the magnetic flux must be concentrated. In this case, distinction is made between magnetic circuits in which the magnetic return path is produced through a yoke of soft magnetic material, and magnetic circuits which make do without a yoke. The latter are usually variants of so-called Halbach magnets, in which the return flux is simplified by a gradual variation of the magnetization direction. In practice, a Halbach arrangement is typically achieved by a stepwise variation of the magnetization direction. This departure from the ideal Halbach magnetization, and in particular also the finite length of the Halbach arrangement, contribute to an increase of the stray field outside the magnet arrangement per se, and make additional shielding necessary for stray field-free applications. In the case of yoke-based magnets, sufficient shielding of the stray field can be achieved by skillful yoke design and use of the yoke material far below the saturation flux density.
In order to achieve the required field homogeneity in the sample volume, in the Halbach embodiments described it is generally necessary to provide correction mechanisms in order to be able to compensate for tolerances of the magnet material or of the positions of the individual magnet units, which complicates the mechanical structure.
One problem then consists in determining the dominant perturbing field orders and establishing a causal relationship with a corresponding adjustment mechanism.
Naturally, it is the case that any variation of the spatial arrangement of the magnets has an effect on all the field orders. Correspondingly, an arrangement in which all the magnet segments are adjustable degenerates into a “multidimensional” optimization problem with an enormous number of degrees of freedom. This means that the “result vector” for a particular “shim target” is very long; it would in essence be necessary to carry out correction on very many of these adjustment elements. Not only is this balancing process very time-consuming, but it is not a priori certain that such a procedure will in fact ever converge to an absolute optimum.
There are orders for which correction has a higher priority, predominantly the “zonal” terms and linear gradients. It is thus necessary to find geometrical variations which can be connected with the prioritized orders, preferably independently of one another. These variations should be as technically simple as possible to implement, since it is generally necessary to work against magnetic forces, and the adjustment precision in very complex movement procedures usually suffers, or its technical achievement becomes exorbitantly expensive. Linear movement and rotation can usually be carried out with manageable outlay.
The permanent magnets are typically held by a supporting structure, which may additionally fulfill the function of the magnetic return path. Traditional designs have a yoke in the form of a rectangular frame (window frame, H yoke, C yoke) with two central magnet modules lying opposite one another, between which the sample volume is located. Because of their open layout, these yoke designs also usually suffer from stray fields which extend far beyond the outer contours of the magnet arrangement.
In order to achieve the field homogeneity in the sample volume which is required for NMR measurements, in the case of yoke-free Halbach magnets it is necessary to provide correction mechanisms in order to be able to compensate for tolerances of the magnet material or the position of the individual magnet units, which complicates the mechanical structure. Yoke-based magnets generally have a parallel pole piece pair consisting of a soft magnetic material, with a correspondingly high saturation flux density. By suitable selection of the pole shoe geometry and special surface processing, the field profile can be optimized in a relatively simple and efficient way.
U.S. Pat. No. 8,077,002 B2 discloses a permanent magnet device for MRI applications. It comprises a pair of solid disk-like magnets, which delimit the air gap as a symmetry plane in a parallel manner and each have an offset (protruding) ring magnet, so that an L-shaped magnet is formed. Pole shoes are arranged on both sides of the air gap for parallel alignment of the magnetic field in the air gap. The so-called L-magnet arrangement is held by a T yoke. Since this magnet arrangement is an open structure, this T yoke is necessary because of the way it is designed. The T yoke sections described are connected by an additional strut as a magnetic return path.
U.S. Pat. No. 7,084,633 B2 discloses a magnet arrangement for MRT instruments using permanent magnets. The permanent magnets are arranged in such a way that a pair of central permanent magnets are respectively connected to a pole shoe, in the intermediate space of which the measurement volume is located. The magnetizations of the two magnets are oriented in the same direction, so that the magnetic flux flows in a defined direction through the measurement volume. Further segments of permanent magnets are arranged in the shape of a circle around said magnets, the magnetization directions of the magnets being directed radially outward or radially inward, so that the magnetic field inside the measurement volume is strengthened. Plates consisting of ferromagnetic material are arranged as a yoke for the return of the magnetic field. For design reasons and in order to guide the magnetic flux of the corner segments, supporting yokes are required at the respective corners.
US 2012/0013338 A1 discloses a magnet arrangement for magnetic resonance devices, which consists of at least two rings with different magnetization directions. In one embodiment, a central Halbach ring (see FIG. 13 therein) is arranged, which is flanked by two radially magnetized rings. The purpose of the known device is to obtain a magnetic field which is adjustable in the magnetization direction, and which can be varied from 0° to 90°, and in particular also around the magic angle of 54.7°. The magnet segments are likewise intended to be orientable, so that a maximally homogeneous field is formed in the measurement volume. A disadvantage with this, however, is the complicated structure, which is difficult to produce in practice.
EP 1 876 462 A1 discloses a stack of at least two magnet rings, which are arranged around a sample space and are mounted rotatably relative to one another. The individual rings preferably have a Halbach geometry. By rotation of the rings, it is possible to achieve a field sweep, such as is used in ESR technology. The focus of this arrangement is not to be able to vary the nominal strength of the magnetic field during operation, but to improve the homogeneity. To this end lateral displacements of the rings are essential, and although axial rotatability is provided it is only of secondary importance in this case. In EP 1 876 462 A1, a concentric arrangement is prescribed. By definition, there is concentricity when the midpoints are identical, but this is not the case for rings which are arranged along a common axis. The magnet groups described in EP 1 876 462 A1 are displaceable neither along the z axis nor transversely thereto. The only degree of freedom claimed is restricted to the mutual rotatability. This is possible in the known arrangement precisely because the magnet groups are arranged coaxially, i.e. they have the same rotation axis.
U.S. Pat. No. 4,355,236 and U.S. Pat. No. 4,862,128 also disclose permanent magnet rings in Halbach configuration (QUADRUPOLE) as a stack of a plurality of rings, which are mounted rotatably relative to one another so that the magnetic flux in the measurement volume can be varied. The rings are not, however, displaceable relative to one another eccentrically in the x-y plane, and also cannot be spaced apart in the z direction.
U.S. Pat. No. 4,355,236 describes a multipole magnet, and in particular reference is exclusively made to quadrupoles, the reason for which is the intended use as a focusing element for a charged particle beam. This is because quadrupoles constitute the best focusing element in particle accelerators (see also FODO structure).
To this end, the individual magnetic rings must be rotatable, in order to also be able to cancel a field when necessary, and rotatability even up to 45° is consequently necessary. Furthermore, the particle beam, in particular electron beam to be focused, is very small in comparison with a conventional MR measurement volume. Therefore, the requirements for the homogeneity of the arrangement according to U.S. Pat. No. 4,355,236 are also not nearly as demanding as in the case of an MR dipole. The magnet groups described in U.S. Pat. No. 4,355,236 are displaceable neither along the z axis nor transversely thereto. U.S. Pat. No. 4,355,236 it is therefore to be regarded as rather remote in relation to the generic arrangement defined in the introduction, especially because possible applications for NMR or EPR are not mentioned at all in U.S. Pat. No. 4,355,236.
The disclosure content, relevant for the present invention, of U.S. Pat. No. 4,862,128 is very similar to that of the above-described U.S. Pat. No. 4,355,236. U.S. Pat. No. 4,862,128 does not, however, define a particular field of application, but merely the “generation of a homogeneous magnetic field”. The magnet groups here are also displaceable neither along the z axis nor transversely thereto. The point in question is a linear array, but not displaceability for producing or improving the homogeneity.
EP 2 144 076 B1, which was cited in the introduction, discloses a Halbach architecture consisting of three rings, a central magnet ring being flanked by two head rings. The rings are mutually displaceable in the longitudinal direction by using screws or threaded nuts for the purpose of field homogenization, as revealed by paragraphs [0021]-[0025] and [0040], as well as FIG. 7 therein. EP 2 144 076 B1 furthermore discloses that the rings consist of individual segments, which are alternately trapezoidal and rectangular, the individual segments being displaceable in the radial direction for field homogenization. This magnet is constructed according to EP 2 144 076 B1, and has 64 mechanical degrees of freedom. To adjust these is in any event quite difficult. Furthermore, the arrangement according to EP 2 144 076 B1 does not consist of Halbach rings premounted in a fixed fashion, which are produced from trapezoidal individual component parts. The rings are also not displaceable relative to one another in the transverse direction, or through rotational movement. A disadvantage is therefore that the structure and the mounting are very complicated and expensive. The arrangement is not even likely to achieve field strengths of 1.9 T, since, due to the mechanical structure, the entire volume cannot be used. Explicitly, EP 2 144 076 B1 in any case discloses merely a maximum field strength of 0.7 T.